Anatomically, the bony labyrinth in the petrous part of the temporal bone consists of three continuous fluid-filled portions. These areas are the large vestibule, and the three semicircular canals and the cochlea, which arise from the vestibule. All three continuous bony components contain perilymph, a fluid similar to cerebrospinal fluid (CSF), from which it may be at least partly derived.
The vestibule is an irregular, oval space, approximately 3 mm in diameter, that communicates with the cochlea rostrally and with the semicircular canals caudally. The walls of the vestibule are marked by depressions and ridges that correspond to the various portions of the enclosed membranous labyrinth. The medial wall contains two depressions: Caudodorsal is the elliptical recess, which contains the utricle, and rostroventral to it is the spherical recess for the saccule. The vestibular crus separates the two recesses. Several groups of small openings that accommodate the nerves of this region occur near the recesses. These tiny groups of foramina are called maculae cribrosae. In the vestibule are two openings: the more dorsal vestibular window in which is inserted the foot plate of the stapes and the more ventrorostral cochlear window, which is closed by a membrane and is located at the end of the cochlea where perilymph vibrations can be dampened into the tympanic cavity.
There are three semicircular canals, an anterior, a posterior, and a lateral canal. They lie caudal and slightly dorsal to the vestibule. Each canal describes approximately two thirds of a circle in a single plane, and each is approximately at a 90-degree angle to the other two. The segment of the canal that communicates with the vestibule is called the crus. Each canal has two crura that communicate with the vestibule (with the exception of the common crus, to be noted later). One crus of each canal has a dilation, the osseous ampulla (ampullae osseae) near the junction with the vestibule. The lumen diameter of the canals averages roughly 0.5 mm, the ampulla being approximately twice as large.
The anterior canal of one ear is roughly parallel with the posterior canal of the opposite ear. The lateral canal of each side occupies a nearly horizontal plane. The anterior canal is the longest. The arc it forms measures approximately 6 mm across at the widest part. The lateral canal forms an arc that measures approximately 4.5 mm, while the arc of the posterior semicircular canal is the smallest, measuring only 3.5 mm in medium-sized dogs. These measurements vary with the size of the dog. The common crus is formed by the nonampullated ends of the posterior and anterior canals. In sculptured specimens the anterior semicircular canal is seen to surround the floccular fossa, a small but deep depression on the medial side of the petrous part of the temporal bone. This depression is occupied by the paraflocculus of the cerebellum. The ampullated end of the posterior canal and the nonampullated end of the lateral canal are united for a short distance caudal to the vestibule.
The cochlea is the bony shell that surrounds the cochlear duct in a spiral of three and one-quarter turns around a central hollow core of bone, the modiolus, which contains the cochlear nerve and blood vessels. The cochlea points ventrorostrally and slightly laterally within the promontory of the petrous part of the temporal bone. The osseous spiral lamina that winds around the modiolus, much like the thread of a screw, nearly bisects the lumen of the cochlea into two portions, called the scala tympani and scala vestibuli. The osseous spiral lamina begins within the vestibule and ends at the apex in a free hooklike process, the hamulus. The scala vestibuli communicates with the vestibule, and hence the perilymph within it is acted on by the vibrations of the base of the stapes in the vestibular window. The cochlear window is an opening situated near the rostral end of the vestibule by which the scala tympani communicates with the tympanic cavity. A secondary tympanic membrane closes this cochlear window. The membranous cochlear duct, formerly scala media, completes the separation of the two scalae. The scalae communicate at the apex of the modiolus by a small opening, the helicotrema, formed at the free border of the hamulus. The basal turn of the cochlea is approximately 4 mm in diameter and lies close to the medial side of the vestibule. cochlea measures approximately 7 mm. Longitudinal modiolar canals and a spiral modiolar canal serve for the distribution of both blood vessels and nerves to the cochlea. The source of perilymph may depend on its location. One source is CSF from the subarachnoid space that gains entrance to the scala tympani of the cochlea via the perilymphatic duct (ductus perilymphaticus) (not part of the membranous labyrinth) in the small cochlear canaliculus (canaliculus cochleae). This small canal courses directly ventrad from a point on the ventral wall of the scala tympani near its origin to communicate with the subarachnoid space. The other source is as an ultrafiltrate from the cochlear blood vessels in the modiolus into the scala vestibuli.